Judy L. Stone

Heritability of stigma position and the effect of stigma-anther separation on outcrossing in a predominantly self-fertilizing weed, Datura stramonium (Solanaceae)

A polymorphism for anthocyanin production was used as a genetic marker to document the relationship between anther-stigma separation and outcrossing rate in the predominantly self-fertilizing weed Datura stramonium. White-flowered plants that differed in anther-stigma separation were placed into populations consisting exclusively of purple-flowered plants. Self vs. outcross origin of progeny was evident in the hypocotyl color of the seedlings. Outcrossing rates measured for single flowers were significantly positively correlated with anther-stigma separation, albeit with some scatter around the regression line, especially for flowers with exserted stigmas. We also performed an 8 × 8 diallel cross to determine whether anther-stigma separation is genetically determined. Heritability in two field plots was ∼0.3 and in the greenhouse was ∼0.2. Maternal effects, epistasis, and dominance appeared to be relatively unimportant. Genotypes performed consistently across the three environments, although total plant size varied more than fivefold. It appears that the mixed-mating system of D. stramonium has a heritable basis and would be capable of responding to selection.

Competition among old-field perennials at different levels of soil fertility and available space

Competitive effects and responses to neighbours were examined by growing plants of three species (Achillea millefolium, Dactylis glomerata and Vicia cracca) singly and in intra- and interspecific pairs at low and at moderate substrate fertility. The effects of neighbours were distinguished from the effects of a reduction in available space by growing plants across a range of pot sizes. Focal plants were affected by a simple reduction in available space differently than by the presence of competing neighbours (...)

THE EVOLUTION OF DISTYLY : POLLEN TRANSFER IN ARTIFICIAL FLOWERS

A recent model by Lloyd and Webb derives conditions necessary for the evolution of distyly based on pollen‐transfer probabilities between ancestral morphs and invading mutants. We used bumblebees visiting artificial flowers to measure the parameters of the model. Our findings supported the first evolutionary step proposed by the model, establishment of a stigma‐height polymorphism. Conditions for the subsequent establishment of an anther‐height polymorphism were not satisfied by pollen‐transfer patterns alone. Because conditions for the first step are considered more onerous, however, and because the second stage depends on inbreeding depression as well as pollen‐transfer patterns, we interpret our results as supporting the plausibility of the Lloyd‐Webb model. Video images of bees visiting glass‐sided artificial flowers demonstrate a mechanism for disassortative pollination between the ancestral and mutant morph. In general, pollen‐transfer probabilities were negatively correlated with the height difference between anthers of the donor and stigma of the recipient. Style length affects bee feeding posture in such a way that disassortative pollination could feasibly occur in the absence of an anther‐height polymorphism.

Molecular Mechanisms Underlying The Breakdown Of Gametophytic Self‐incompatibility

The breakdown of self‐incompatibility has occurred repeatedly throughout the evolution of flowering plants and has profound impacts on the genetic structure of populations. Recent advances in understanding of the molecular basis of self‐incompatibility have provided insights into the mechanisms of its loss in natural populations, especially in the tomato family, the Solanaceae. In the Solanaceae, the gene that controls self‐incompatibility in the style codes for a ribonuclease that causes the degradation of RNA in pollen tubes bearing an allele at the S‐locus that matches either of the two alleles held by the maternal plant. The pollen component of the S‐locus has yet to be identified. Loss of self‐incompatibility can be attributed to three types of causes: duplication of the S‐locus, mutations that cause loss of S‐RNase activity, and mutations that do not cause loss of S‐RNase activity. Duplication of the S‐locus has been well studied in radiation‐induced mutants but may be a relatively rare cause of the breakdown of self‐incompatibility in nature. Point mutations within the S‐locus that disrupt the production of S‐RNase have been documented in natural populations. There are also a number of mutants in which S‐RNase production is unimpaired, yet self‐incompatibility is disrupted. The identity and function of these mutations is not well understood. Careful work on a handful of model organisms will enable population biologists to better understand the breakdown of self‐incompatibility in nature.

Variation in the self-incompatibility response within and among populations of the tropical shrub Witheringia solanacea (Solanaceae).

Breakdown of genetically enforced self-incompatibility (SI), an extremely common and important evolutionary transition in plants, has conventionally been conceived as a qualitative rather than a quantitative change. We evaluated qualitative and quantitative variation in SI for four populations of Witheringia solanacea in Costa Rica, examining growth of self-pollen tubes in pollinations of buds and mature flowers. We also measured levels of RNase production in styles to determine whether enzyme production was correlated with differences in self-rejection. The two small populations contained both self-compatible (SC) individuals and obligate outcrossers (female or SI). Plants in the two large populations were uniformly SI as revealed by pollen tube growth, although several of these individuals sporadically set seed autogamously. Stylar RNase activity did not differ significantly between bud and mature flowers, but self-pollen tube growth did differ, suggesting that a gene product in addition to S-RNase is responsible for developmental onset of SI. Population-level differences in RNase activity were consistent with differences in the strength of the rejection response in bud pollinations, suggesting that a threshold level of S-RNase, in combination with other factors, is necessary for SI. Our results support a growing body of evidence that not only qualitative variation in SI, but also quantitative variation may be functionally significant.

Genetic Mosaics in Strangler Fig Trees: Implications for Tropical Conservation

Single trees of six species of strangler figs (Ficus spp., Moraceae) in Panama were found to be made up of multiple genotypes, presumably formed by the fusion of different individuals. The phenomenon is frequent enough that strangler fig populations will contain considerably more genetic variation than would be expected from the number of trees. How this cryptic variation affects populations depends on the flowering phonology of composite trees. If the genetically different portions of trees flower asynchronously, populations of pollinating wasps may be more resistant to low host population sizes than previously thought. If different portions flower synchronously, attempts to infer mating-system parameters from the parentage of fruit crops will be misleading. The fruiting of figs, which are considered a keystone species in tropical forests, is important for maintaining biodiversity but is also particularly susceptible to failure at small population sizes. It is therefore important to know both the number of trees and the number of genotypes in a population.

Transmission advantage favors selfing allele in experimental populations of self-incompatible Witheringia solanacea (solanaceae).

The evolution of self‐fertilization is one of the most commonly traversed transitions in flowering plants, with profound implications for population genetic structure and evolutionary potential. We investigated factors influencing this transition using Witheringia solanacea, a predominantly self‐incompatible (SI) species within which self‐compatible (SC) genotypes have been identified. We showed that self‐compatibility in this species segregates with variation at the S‐locus as inherited by plants in F1 and F2 generations. To examine reproductive assurance and the transmission advantage of selfing, we placed SC and SI genotypes in genetically replicated gardens and monitored male and female reproductive success, as well as selfing rates of SC plants. Self‐compatibility did not lead to increased fruit or seed set, even under conditions of pollinator scarcity, and the realized selfing rate of SC plants was less than 10%. SC plants had higher fruit abortion rates, consistent with previous evidence showing strong inbreeding depression at the embryonic stage. Although the selfing allele did not provide reproductive assurance under observed conditions, it also did not cause pollen discounting, so the transmission advantage of selfing should promote its spread. Given observed numbers of S‐alleles and selfing rates, self‐compatibility should spread even under conditions of exceedingly high initial inbreeding depression.

Rare Estuary Monkeyflower in Merrymeeting Bay is Genetically Distinct

Abstract A common garden experiment was used to test for genetic differentiation between populations of the widespread Mimulus ringens var. ringens and the very locally distributed var. colpophilus, in and around Merrymeeting Bay in Maine. Measurements in the field documented a statistical difference for leaf length and calyx lobe length between populations found in non-tidal versus tidal waters, although most populations surveyed fell within values originally established for var. ringens. Field-collected seeds grown in the greenhouse showed differentiation between varieties for flowering date and for three of four of the morphological characters measured. A second generation grown in the greenhouse showed differentiation between varieties for the same three morphological characters, but not for flowering date.

Embryonic inbreeding depression varies among populations and by mating system in Witheringia solanacea (Solanaceae).

UNLABELLED PREMISE OF THE STUDY Embryonic inbreeding depression is a key influence on mating system evolution and can be difficult to estimate in self-incompatible species. A pollen chase experiment was used to estimate the magnitude of embryonic inbreeding depression in Costa Rican Witheringia solanacea, a species polymorphic for self-incompatibility (SI). In a pollen chase experiment, bud self-pollinations are followed after anthesis by outcross pollinations, with a comparable pair of outcross pollinations used as a control. Lowered seed set for the self-precedence treatment indicates embryonic inbreeding depression. • METHODS Embryonic inbreeding depression was assayed for self-compatible (SC) individuals and for SI plants from two populations that differ quantitatively in the onset and enzymatic activity of their SI response. Microsatellite markers were used to assay the selfing rate of a sample of surviving progeny from the prior self-pollination treatment. • KEY RESULTS SC individuals showed no evidence of embryonic inbreeding depression. In SI plants, prior self-pollination reduced seed number by 28-70%, depending on population. Microsatellite genotyping revealed that embryonic inbreeding depression was even more severe than estimated by the phenotypic data: for mature fruits resulting from self-pollination precedence, the majority of the progeny were the result of outcross fertilization. • CONCLUSIONS Lineage-specific purging of recessive lethals has accompanied the evolution of SC in this species. SI populations show contrasting levels of embryonic inbreeding depression, with nearly complete embryonic lethality upon selfing in the Monteverde population. In the face of high embryonic inbreeding depression, an increase in selfing rate can evidently occur only under severe pollen limitation.